The Pleiades Star Cluster, M45, from a Light Polluted Metro Area

The Pleiades star cluster, also known as the Seven Sisters is immersed
in nebulosity due to scattered starlight. This image was made from the
Denver, Colorado metro area, a region of high light pollution. An image
with the light pollution is shown below. The light pollution was
subtracted in post processing to reveal the beautiful blue nebulosity
and many faint and colorful stars. The blue color is due to Rayleigh
scattering by dust particle smaller than the wavelength of visible
light. Rayleigh scattering is the reason our daytime sky is blue: scattering off
of air molecules.

Technical.
Canon 7D Mark II 20 megapixel digital camera
and
Canon EF 200mm f/2.8L II USM Lens
plus a 1.4x teleconverter, giving 230 mm at f/4.
(I apparently changed the zoom point during focusing--I intended to be at 280 mm.)
A total of 32 one-minute exposures at ISO 1600 were added (32 minutes total exposure).
No dark frame subtraction, no flat fields, no noise reduction.
Tracking with an astrotrac. Crop to central 1/3 of the image.

Modern DSLRs like the
Canon 7D Mark II
include on sensor dark current suppression
and low fixed pattern noise at ISOs around 1600 and higher,
making no need for dark frame subtraction. Modern raw converters
correct for light fall-off and also correct for hot/dead/stuck pixels.
This makes processing low light images easy: simply align and
average. Compare this image to the M45 made with a 7D Mark 1 with almost
the same exposure time and almost the same sky conditions.

The Exposure Factors, CEF, CEFA
are measures of the relative amounts of light received from a subject.
It can be used to fairly compare wildly different lens/telescope apertures and exposure times.
For this image: